1. Field of the Invention
The present invention relates generally to an active noise control system for causing noises produced from a noise source to interfere with a control sound so as to decrease the noise. More specifically, the invention relates to an active noise control system which can be applied to decrease noises in a passenger cabin of an automotive vehicle, an airplane and so forth.
2. Description of The Background Art
One of active noise control systems of this type has been disclosed in British Patent First (unexamined) Publication No. 2149614.
This conventional system may be applied to a closed space, such as a passenger cabin of an airplane, and may operate only under a condition in that a single noise source (a primary sound source), such as an engine arranged outside the closed space, produces sounds including a fundamental blade frequency f.sub.0 and higher harmonic waves f.sub.1 to f.sub.n. To be concrete, this conventional system comprises a plurality of loudspeakers (secondary sound sources) and microphones arranged within the closed space; frequency detecting means for detecting frequencies f.sub.0 to f.sub.n of noise sources; and a signal processor for supplying signals having opposite phases to the detected frequencies f.sub.0 to f.sub.4, to the loudspeakers on the basis of the output signals of the microphones and the detected signals of the frequency detecting means. With this construction, this conventional system can minimize the sound pressure level within the closed space by causing the secondary sounds produced from the loudspeakers to interfere with the primary sound transmitted from the noise sources.
However, since the aforementioned conventional active noise control system is designed to decrease noises within the closed space produced from a single noise source (a primary sound source), there is a disadvantage in that, for example, when noises are transmitted from a plurality of noise sources at the same time, any one of the noise sources must be selected as the detected object of the frequency detecting means, so that the system can not effectively decrease the noises from the plurality of noise sources. In addition, if a plurality of the aforementioned conventional systems are provided for a plurality of noise sources and are designed to operate independently of each other, there is a disadvantage in that the system becomes expensive and cannot effectively control sound reduction since the number of operational elements is limited.